The present invention relates to display devices and, more particularly, to an improved backlight system for liquid crystal device ("LCD") displays, designed for use in daytime and with night vision imaging systems.
1. Review of the Prior Art.
LCD's have been employed as modulators of light, and, as such, have been extensively employed in various devices as displays for information in various forms and formats. Reflective LCD's rely upon impinging illumination which enters the face of the device, is reflected from a rear surface, and exits the face. Reflective LCD's have the advantages of lower power requirements, and reductions in size and weight compared to transmissive LCD's, which must have illumination sources.
Typical uses of reflective LCD's are in wrist watch faces and some lap top computer screens. These require ambient light to provide the contrast between the light and dark portions of the display. Reflective LCD's do not work effectively in a dimly lighted environment, and are completely ineffective in darkness. Some watchmakers solve that problem by providing a small light source in the watch to illuminate the face, creating the necessary contrast between light and dark. However, some of the benefit of the reflective device (i.e., lower power and fewer components) is lost thereby.
While suffering in the presence of low ambient illumination, the reflective device is also sensitive to the light transmissive ability of the LCD, since the luminance of the display is dependent upon the square of the transmission of light through the LCD. This is because the ambient light must travel through the LCD to the reflective surface and then back ,out through the LCD again.
Typically, a monochrome twisted nematic LCD, such as are used in watch faces, can have light transmission of up to 35%. This is adequate for reflective mode viewing, where the luminance level of the display will be about 10% of the viewing ambient.
In contrast, Full Color, LCD displays must use artificial illumination devices due to the extremely low rate of light transmission through the display. Creating a color display requires the incorporation of absorptive color dye filter mosaics along with polarizers, diffusers and other absorptive structures. The absorptive dyes and other structures allow light transmissions in the range of less than 5%, and in the reflective mode would have a luminance of approximately 0.25% of the ambient light level, which is unacceptably low.
A typical backlight illumination source is a fluorescent lamp, which has the requisite properties of high efficiency, long life and good color balance (i.e., transmits the full range of frequencies within the visible spectrum in their normal concentrations). Fluorescent lamps are also capable of dimming over a broad dynamic range (typically up to 2,000:1), however a wider range of luminance levels might be needed for displays that are to be viewed in total darkness as well as in high ambient illuminations.
Transmissive color LCD displays have found application in color television projection systems and in "flat screen" color displays for computer systems. Yet another application for such devices is in avionics multi-function displays, which are required to present both full-color graphics and multi-gray scale video imagery. Such a display must have high brightness and contrast for good sunlight viewing, as well as a wide dimming range for night time operation.
Sanai et al., U.S. Pat No. 5,029,045, Horiuchi, U.S. Pat. No. 4,998,804, and Suzawa, U.S. Pat. No. 4,618,216 teach the full-color illumination of LCD's, while Tanaka et al., U.S. Pat. No. 5,040,098, allows illumination of monochromatic LCD displays whose color attribute is adjusted by means of organic fluorescent dyes. The LCD illumination device of Hamada, U.S. Pat. No. 4,914,553 utilizes a Fresnel reflection surface, and can be used with either built-in or external light sources. Masuzawa et al., U.S. Pat. No. 4,642,736, teaches a light diffuser for normal illumination of various types of machinery as well as LCD devices.
In an earlier patent of the present inventor, U.S. Pat. No. 5,211,463, issued May 18, 1993, and assigned to the assignee of the present invention, several back light devices were disclosed that could be used with night vision imaging systems ("NVIS"). Several alternative embodiments were shown in which the night illumination source was either chosen from a class of sources that did not emit infrared and near infrared frequencies, or a filter that removes infrared and near infrared frequencies was interposed between the night light source and the remaining structure.
During the prosecution of the applications, the following references were cited as being relevant, but were not applied: U.S. Pat. Nos. 5,146,354; 5,143,433; 4,998,804; 4,934,793; and 4,277,817. Accordingly, no discussion of these references is included herein.
The primary object of the present invention is to permit the reading of an LCD display at night with minimal luminance (to preserve dark adaptation), as well as in daylight conditions. It appeared that the structures of the prior U.S. Pat. No. 5,211,463 could be adapted to this task by removing the infrared blocking filters.